Partial sequence of human complement component factor B: novel type of serine protease

Human complement factor B: functional properties of a recombinant zymogen of the alternative activation pathway convertase

The human complement factor B is a centrally important component of the alternative pathway activation of the complement system. Here we report the isolation, characterization and eukaryotic expression of the first full length cDNA transcript for human factor B. In a factor B dependent haemolysis assay, the recombinant human factor B generated by transient COS cell transfection was shown to reconstitute haemolytic activity of factor B depleted human serum. To study the biological activities assigned to factor B, the availability of recombinant polypeptides representing definite portions of the human factor B molecule is desirable.

The Bb fragment of complement factor B acts as a B cell growth factor

The process of B cell growth and differentiation into plasma cells is highly regulated and may be influenced by a large number of inflammatory mediators, including complement components. We have studied the regulatory influence of Bb, a 60-kD peptide created during the cleavage of complement Factor B by Factor D and C3b. Purified Bb alone had no effect on proliferation and differentiation of human splenic or tonsillar B cells. However, when B cells were activated by Staphylococcus aureus Cowan I (SAC), Bb enhanced proliferation in a dose-dependent manner. Bb also enhanced proliferation when cocultured with SAC and suboptimal concentrations of purified 60-kD B cell growth factor (HMW-BCGF), a previously described lymphokine that is known to possess growth-promoting activity. However, Bb had no effect on cells treated with optimal concentrations of HMW-BCGF. Like HMW-BCGF, Bb's major effect was on the larger in vivo activated B cells. Half-maximal enhancement of proliferation was reached at a Bb concentration of 1-10 nM. Of note is the fact that antibody to Factor B recognized HMW-BCGF, and an mAb to HMW-BCGF also recognized Factor B and Bb, but not Ba. Moreover, radiolabeled Bb bound saturably to activated B cells and to an EBV-transformed human B cell line. The binding of Bb was inhibited by HMW-BCGF but not by Ba or IgG. Thus, Bb is antigenically and functionally related to HMW-BCGF, and can act as a B cell growth and differentiation factor at potentially physiologic concentrations. These data suggest that Bb may be important in amplifying the immune response in areas of inflammation. Since complement activation occurs at inflammatory sites long before induction of HMW-BCGF synthesis, Bb may be an early signal for the clonal expansion of antigen-activated B cells.

Primary structure of human complement component C2. Homology to two unrelated protein families

The primary structure of the second component of human complement (C2) was determined by cDNA cloning and sequence analysis. C2 has 39% identity with the functionally analogous protein Factor B. The C-terminal half of C2a is homologous to the catalytic domains of other serine proteinases. C2b contains three direct repeats of approx. 60 amino acid residues. They are homologous to repeats in Factor B, C4b-binding protein and Factor H, suggesting a functional significance of the repeat in C4b and C3b binding. The repeats are also found in the non-complement proteins beta 2-glycoprotein I and interleukin-2 receptor, and this repeat family may be widespread.

Molecular biology of the human and mouse MHC class III genes: phylogenetic conservation, genetics and regulation of expression

The generation of complementary and genomic DNA clones for the human and mouse MHC class III genes has advanced the study of the organization, structure, genetics and expression of these loci. These clones have been useful in defining new polymorphic markers in each species and therefore permit a more complete genetic analysis of the complement cluster and the MHC as a whole. The coding sequences of the factor B and C4 genes are extensively conserved both within and between species, in contrast to the coding sequences of other MHC products. In human and mouse, the organization of the class III genes is similar with respect to order and position between the class II and class I regions of the MHC. However, these inter-species similarities in the organization and products of the class III genes does not extend to their regulation. In addition to complement gene expression being regulated differently between tissue sites within a species, expression is apparently regulated differently in analogous tissues between species. The considerable progress which has been made in the molecular analysis of C2, factor B and C4 using DNA clones forms the basis for the future study of the biology of the class III genes and the role of complement in inflammatory processes and in the immune system.

The structure and genetics of the C2 and factor B genes

This review summarises our current knowledge of the genetic organisation, structure and polymorphism of the loci for the complement proteins, C2 and Factor B--class III gene products of the major histocompatibility complex. cDNA probes specific for C2 and Factor B have been used to screen cosmid libraries of human genomic DNA, and this has allowed isolation and characterisation of the corresponding genes. Southern blot analysis of the cosmid clones and of uncloned genomic DNA has shown that there are single C2 and Factor B loci that are less than 500 bp apart. Molecular mapping has revealed that the C2 gene spans approximately 18 kb of DNA. This is in marked contrast to the Factor B gene which is 6 kb in length. The entire gene structure of the Factor B gene has been determined and the interesting features of this gene which have emerged from an examination of the intron-exon boundaries are discussed. C2 and Factor B are polymorphic and structural variants have been detected by differences in charge. The degree of polymorphism at the C2 and Factor B loci has been examined by Southern blot analysis of restriction digests of genomic DNA. Three DNA polymorphisms have been identified in the C2 gene. These polymorphisms subdivide the common allelic variant of C2 (C2C) and reveal that there is much greater variability at the C2 locus than that detected by protein typing. It is suggested that these DNA polymorphisms may serve as useful markers in the genetic analysis of diseases that are related to the major histocompatibility complex.

Structure and activation of complement components C2 and factor B

The activation of complement is initiated by two independent pathways. Each leads to the formation of a complex protease, C3 convertase, with equivalent specificity and function but different composition. The convertase derived from the classical pathway is composed of complement components C4 and C2 while that from the alternative pathway consists of components C3 and Factor B. C2 and Factor B contain the catalytic site of each convertase respectively. The amino acid sequence of Factor B has been determined. Limited sequence of CNBr-peptides isolated from C2 has also been obtained. The two enzymes are shown to be homologous and to represent a novel type of serine proteinase, characterized by their unusual structure and mechanism of activation, when compared to known serine proteinases.

Chromosomal assignments of human genes for serine proteases trypsin, chymotrypsin B, and elastase

The genes for the serine proteases trypsin, chymotrypsin B, and elastase were chromosomally assigned in man using cDNA probes that have been isolated from a rat pancreatic cDNA library. DNA from human X rodent somatic cell hybrids was cleaved with BamHI or EcoRI and analyzed by Southern filter hybridization methods for the segregation of the genes for trypsin-1 (TRY1), chymotrypsin B (CTRB), and elastase-1 (ELA1). TRY1 was assigned to human chromosome 7q22----qter, CTRB to chromosome 16, and ELA1 to chromosome 12. Although the three genes are members of the same gene family, they are dispersed over different chromosomes.

The effects of iodine and thiol-blocking reagents on complement component C2 and on the assembly of the classical-pathway C3 convertase

I2 can react with complement component C2 in a two-stage process. In the first stage, a form of C2 with enhanced haemolytic activity is produced. This form of C2 is cleaved to C2a and C2b by C1s at the same rate as native C2. The enhanced C2 haemolytic activity correlates with the ability to form a stable fluid-phase C3 convertase on addition of the C2 to C4b and C1s. It reflects an increased affinity for C4b of C2a formed from I2-treated C2, although the affinity for C4b of I2-treated C2 itself is not markedly increased. The specific activity of C3 convertase formed from I2-treated C2 is the same as that formed from native C2. The second stage of the reaction with I2, which is favoured at high pH or in the presence of excess I2, inactivates C2 on production of a species that cannot be cleaved by C1s. The presence of a single free thiol group in C2, which is the site of modification by I2, was confirmed by titration with p-chloromercuribenzoate, iodoacetamide and 5,5'-dithiobis-(2-nitrobenzoic acid). A single thiol group is also present in Factor B, and the cysteine residue, like that in C2, requires denaturation of the protein before reaction with iodoacetamide and 5,5'-dithiobis-(2-nitrobenzoic acid) but not p-chloro- mercuribenzoate .

The complement components of the major histocompatibility locus

Polymorphism of complement components, recognized by differences in either their antigenic specificity or their electrophoretic mobility, together with studies of inherited deficiencies, has enabled many of their structural genes to be mapped. In humans, three genes (for C2, C4, and factor B) have been placed between HLA-D and HLA-B on chromosome 6 and in mice, C4 between H2-I and H2-D, chromosome 17. Structural studies show that these components have exceptional features. C2 and factor B which contain the proteolytic active site of the C3 and C5 convertases are of the classical and alternative pathway respectively and are similar in structure and function. Both are novel types of serine proteases. C4 (as C3) contains an intrachain thioester bond essential for hemolytic activity. Molecular genetic investigations are determining the relative positions of these genes, and their precise structure, and should clarify their relation to the inherited diseases which are associated with defects in this section of the human genome.

Endogenous inhibitors of factor B

proteins which were able to bind noncovalently with mouse factor B were found in cells that are nonsecretors of factor B such as mouse-established monocytic cells and L cells but not in peritoneal resident macrophages. These proteins were isolated from lysates of L cells and separated into four distinct proteins by preparative SDS-polyacrylamide gel electrophoresis, with molecular weights of 25K , 28K , 33K, and 35K . The individual proteins formed a complex with purified mouse factor B at a molecular ratio of 1: 1 and inhibited its hemolytic activity. Proteins 25K and 28K inhibited the hemolytic activity of an activated form of factor B combined with cobra venom factor as well as that of the native form. These inhibitors did not affect the hemolytic activity of the second component of complement in mouse serum. The inhibitory activity of the 25K protein was partially inhibited by antiserum raised against it in rabbits.

The reaction of iodine and thiol-blocking reagents with human complement components C2 and factor B. Purification and N-terminal amino acid sequence of a peptide from C2a containing a free thiol group

Human complement components C2 and Factor B each contain one free thiol group/molecule. Reaction with p-chloromercuribenzoate destroyed the haemolytic activity of C2 but had no effect on Factor B. Reaction of C2 with I2 gave a 16-fold enhancement of its haemolytic activity. The pH optimum for the reaction was 7.0. The I2 reacted at the thiol group in C2 with a stoicheiometry of 1 mol of I2/mol of C2. The product of the reaction was unaffected by millimolar concentrations of dithiothreitol; however, azide and cyanide were inhibitory. Reaction with azide did not result in re-expression of the thiol group. Mild oxidation of the thiol group with m-chloroperbenzoic acid did not enhance the haemolytic activity. The results suggest that reaction with I2 causes intramolecular covalent, but not disulphide, bond formation. I2 reacted with Factor B at the free thiol group without affecting the haemolytic activity. A CNBr-cleavage peptide from C2a (obtained by cleavage of C2 by subcomponent C1s) containing the free thiol group was isolated. Automated Edman degradation of the peptide showed that it was the N-terminal peptide of C2a. The free thiol group was identified at position 18.

New synthetic inhibitor to the alternative complement pathway

The inhibitory effects of 6-amidino-2-naphthyl 4-guanidinobenzoate (FUT-175) on the activities of factor B, factor D and cobra venom factor (CVF) X Bb were examined. FUT-175 bound specifically to the Bb fragment of factor B or CVF X Bb. FUT-175 was a non-competitive inhibitor of the esterolysis of L-leucyl-L-alanyl-L-arginine naphthylester by factor B and CVF X Bb. FUT-175 also inhibited the haemolytic activity of factor B, the C3 convertase activity of CVF X Bb and the factor B-cleaving activity of factor D. The concentration of FUT-175 causing 50% inhibition of these activities was 10(-5) - 10(-4)M.

Amino acid sequence of the Bb fragment from human complement Factor B. Alignment of the cyanogen bromide-cleavage peptides

The alignment of all the CNBr-cleavage peptides of fragment Bb from human Factor B (a component of the alternative pathway of complement) was determined. This was derived from cleavage of the fragment Bb at arginine residues by using trypsin and clostripain. Details of the isolation and amino acid sequences of these peptides are given. Together with previously published N-terminal sequences of the CNBr-cleavage peptides [Christie & Gagnon (1982) Biochem. J. 201, 555-567], this provides the amino acid sequence of the N-terminal half of fragment Bb.

Amino acid sequence of the Bb fragment from complement Factor B. Sequence of the major cyanogen bromide-cleavage peptide (CB-II) and completion of the sequence of the Bb fragment

The amino acid sequence of peptide CB-II, the major product (mol.wt. 30 000) of CNBr cleavage of fragment Bb from human complement Factor B, is given. The sequence was obtained from peptides derived by trypsin cleavage of peptide CB-II and clostripain digestion of fragment Bb. Cleavage of two Asn-Gly bonds in peptide CB-II was also found useful. These results, along with those presented in the preceding paper [Gagnon & Christie (1983) Biochem. J. 209, 51-60], yield the complete sequence of the 505 amino acid residues of fragment Bb. The C-terminal half of the molecule shows strong homology of sequence with serine proteinases. Factor B has a catalytic chain (fragment Bb) with a molecular weight twice that of proteinases previously described, suggesting that it is a novel type of serine proteinase, probably with a different activation mechanism.

Isolation of cDNA clones for the human complement protein factor B, a class III major histocompatibility complex gene product

cDNA clones corresponding to a major histocompatibility class III antigen, the complement protein factor B, have been isolated from a human adult liver cDNA library. The clones, ranging in size from 1.0 to 2.3 kilobases, were identified by direct hybridization with two synthetic oligonucleotide mixtures. Two regions of the factor B amino acid sequence, each with minimal ambiguity in codon assignment, were chosen for synthesis of the oligonucleotides. The sequences of two clones have been partially determined. They contain coding information for the amino acid sequence of the Bb fragment of factor B and the entire 3' -untranslated region.

The purification and properties of the second component of guinea-pig complement

A method has been developed for the purification to homogeneity of guinea-pig complement component C2. Contrary to previous reports, guinea-pig C2 is a single polypeptide chain with apparent mol.wt. of 102000, the same as human C2. It is cleaved by C1s to yield fragments C2a (apparent molwt. 74000) and C2b (apparent mol.wt. 34000). The amino acid composition and N-terminal sequences of these fragments are similar to those of human C2a and C2b. Human and guinea-pig C2 show more extensive sequence homology to Factor B than previously identified. The known homology around the sites of cleavage by C1s and Factor D has now been extended by a stretch of ten identical or conservatively substituted residues. Sequence homology has now been identified at the N-terminal of C2b and Factor Ba. The properties of the classical-pathway C3 convertases assembled from human C4b, C1s and human or guinea-pig C2 have been compared. The rates of cleavage of human and guinea-pig C2 by C1s (and therefore the rates of assembly of the C3 convertases) are similar. The rate of decay of the activity of the C3 convertase formed from guinea-pig C2 is 10-fold lower than for human C2. This greater stability reflects a higher affinity of guinea-pig C2a for human C4b. The presence of C2b is not necessary for C3 convertase activity.

Purification of human C3b inactivator by monoclonal-antibody affinity chromatography

Monoclonal antibody has been obtained to the human complement control protein C3b inactivator after immunization of mice with the enzyme prepared by conventional methods. Antibody from ascitic fluid was purified and coupled to Sepharose-CL-4B to give a specific affinity column, which was used to isolate C3b inactivator from human serum in 70% yield. The product was characterized by size, chain structure, amino acid analysis and proteolytic activity.

The activation of the alternative pathway C3 convertase by human plasma kallikrein

Human plasma kallikrein can replace factor D for the activation of the alternative pathway C3 convertase of human complement. The factor B cleavage patterns by factor D and kallikrein are indistinguishable. The ability of kallikrein to cleave factor B is influenced by the magnesium ion concentration and the C3b concentration. Factor D is about ten-fold more effective on a molar basis, for the alternative pathway C3 convertase activation than is kallikrein. The physiological role of the action of kallikrein on the alternative pathway C3 convertase is discussed.

Isolation, characterization and N-terminal sequences of the CNBr-cleavage peptides from human complement Factor B. Localization of a free thiol group and a sequence defining the site cleaved by factor D

Nine CNBr-cleavage peptides from Factor B (a component of the alternative pathway of complement) were isolated. Each was characterized by amino acid analysis and automated Edman degradation. One peptide contained a methionyl bond resistant to cleavage by CNBr. The number of CNBr-cleavage peptides is in agreement with the results of amino acid analysis of Factor B and the fragments Ba and Bb. A total of 358 unique residues were identified from the N-terminal sequences of the CNBr-cleavage peptides. These represent approx. 50% and 60% of the total residues of Factor B and fragment Bb respectively. Alignment of two CNBr-cleavage peptides (CB-VIc and CB-IV) provided a continuous segment of 140 residues. This sequence contained the site cleaved by Factor D to generate the Ba and Bb fragments during the activation of complement. Peptide CB-IV contained a free thiol group at a position corresponding to residue 33 of fragment Bb. Amino sugar analyses of Factor B and of fragments Bb and Ba indicated that all the carbohydrate structures of factor B are N-linked to asparagine through N-acetylglucosamine. The two carbohydrate-attachment sites of the Bb fragment were identified.

Characterization of the CNBr peptides generated from the factor B cleavage fragments, Ba and Bb, by molecular exclusion high performance liquid chromatography

Highly purified human factor B of the alternative complement pathway was treated with factor D in the presence of cobra venom factor to generate its Ba and Bb cleavage fragments. These cleavage fragments were isolated by preparative polyacrylamide gradient gel electrophoresis followed by electrodialysis elution and treatment with CNBr. The resultant CNBr cleavage peptides were isolated by molecular exclusion high performance liquid chromatography and characterized by SDS polyacrylamide gel electrophoresis. Results of these experiments indicate that the Ba fragment essentially consisted of a 28,000 CNBr peptide, whereas 34,700 (28,000 + 3,500 when characterized under reducing polyacrylamide gel electrophoresis conditions); 14,500 (=20,000); and 8,300 CNBr peptides were derived from the Bb fragment.

The conversion of human complement component C5 into fragment C5b by the alternative-pathway C5 convertase

The cleavage of human complement component C5 to fragment C5b by the alternative pathway C5 convertase was studied. The alternative-pathway C5 convertase on zymosan can be represented by the empirical formula zymosan--C3b2BbP. Both properdin-stabilized C3 and C5 convertase activities decay with a half life of 34 min correlating with the loss of the Bb subunit. The C5 convertase functions in a stepwise fashion: first, C5 binds to C3b and this is followed by cleavage of C5 to C5b. The capacity to bind C3b is a stable feature of component C5, as C5b also has this binding capacity. Component C5, unlike component C3, does not form covalent bonds with zymosan after activation, and C5 is not inhibited by amines. Therefore C5, although similar in structure to C3, does not appear to contain the internal thioester group reported for C3 and C4.

Human complement component C4. Structural studies on the fragments derived from C4b by cleavage with C3b inactivator

1. One of the activation products of C4, C4b, was prepared, and the reactive thiol group on the alpha'-chain was radioactively labelled with iodo[2-14C]acetic acid. The alpha'-chain was isolated and the N-terminal amino acid sequence of the first 13 residues was determined. 2. C4b was cleaved by C3bINA in the presence of C4b-binding protein and C4d and C4c isolated. The radioactive label and therefore the reactive thiol group were located to C4d. 3. C4c was reduced and alkylated and the two alpha'-chain fragments of C4c were separated. 3. The molecular weights, amino acid analyses and carbohydrate content of the three alpha'-chain fragments were determined. C4d has a mol.wt. of 44500 and a carbohydrate content of 6%. The two alpha'-chain fragments of C4c have mol.wts. of 25000 (alpha 3) and 12000 (alpha 4) and carbohydrate contents of 10 and 22% respectively. 4. The N-terminal amino acid sequences of C4d, the alpha 3 and the alpha 4 fragments were determined for 18, 24 and 11 residues respectively and, by comparison with the N-terminal sequence of the C4b alpha'-chain, the 25000-mol.wt. fragment (alpha 3) was shown to be derived from the N-terminal part of the alpha'-chain. 5. C-Terminal analyses were done on the alpha'-chain and its three fragments. Arginine was found to be the C-terminal residue of C4d and of the alpha 3 fragment. The C-terminal residue of the alpha'-chain and of the alpha 4 fragment could not be identified. The order of the three fragments of the alpha'-chain is therefore: alpha 3(25000)--C4d(44500)--alpha 4(12000). The specificity of C3bINA is for an Arg--Xaa peptide bond.

The separation of functionally distinct forms of the third component of human complement (C3)

Complement component C3 prepared by the method of Tack & Prahl [(1976) Biochemistry 15, 4513-4521] was found to contain the following trace contaminants: C3b, haemolytically inactive C3 with intact alpha- and beta-chains (C3u) and degraded C3 (apparent mol.wt. 140000) with an intact beta-chain but with a fragmented alpha-chain. The proportion of C3u in the C3 is increased on standing and by freezing and thawing. These contaminants could be separated from each other and from native C3 by chromatography on sulphated Sepharose. They have been characterized by their susceptibility to C3b inactivator in the presence of beta 1H, their ability to be cleaved by C3 convertase and their ability to form alternative-pathway C3 convertase in solution. Incubation of C3b or C3u with beta 1H and C3b inactivator resulted in cleavage of the C3 species; the alpha'-chain of C3b was cleaved to fragments of apparent mol.wts. 67000 and 43000, the alpha-chain of C3u was cleaved to fragments of apparent mol.wt. 75000 and 43000. Native C3 and degraded C3 were unaffected by incubation with beta 1H and C3b inactivator. C3u, unlike C3, was not cleaved to C3b by the classical- or alternative-pathway C3 convertase in solution. When C3b or C3 was incubated with factors B and D, forming C3 convertase, the initial rate of factor-B cleavage was several order of magnitude lower in the presence of C3 than in the presence of C3b. The slow rate observed for C3 could be decreased by preincubation with beta 1H and C3b inactivator or by rechromatography of the C3. The degraded C3 did not support factor-B cleavage by factor D.

The Croonian Lecture, 1980. The complex proteases of the complement system

The assembly and activation of the early components of complement, after their interaction with antibody-antigen complexes, are described in terms of the structures of the different proteins taking part. C1q, a molecule of unique half collagen--half globular structure, binds to the second constant domain of the antibody molecules through its six globular heads. A tetrameric complex of C1r2-C1s2 binds to the collagenous tails and leads to formation of the serine-type proteases C1r and C1s. C1s activates C4, which forms a covalent bond between its alpha' chain and the Fab section of the antibody. C2 is also activated by C1s and associates with the bound C4 molecule to form C42, a labile protease that activates C3, but which loses activity as the C2 peptide chains dissociate from C4. C2, by analogy with factor B, the equivalent component of the alternative pathway of activation, appears to be a novel type of serine protease with a similar catalytic site but different activation mechanism to the serine proteases that have been described previously.